MOLECULAR-STRUCTURE AND ENZYMATIC FUNCTION OI LYCOPENE CYCLASE FROM THE CYANOBACTERIUM SYNECHOCOCCUS SP STRAIN PCC7942

A gene encoding the enzyme lycopene cyclase in the cyanobacterium Syne chococcus sp strain PCC7942 was mapped by genetic complementation, clo ned, and sequenced. This gene, which we have named crtL, was expressed in strains of Escherichia coli that were genetically engineered to ac cumulate the carotenoid precursors lycopene, neurosporene, and zeta-ca rotene. The crtL gene product converts the acyclic hydrocarbon lycopen e into the bicyclic beta-carotene, an essential component of the photo synthetic apparatus in oxygen-evolving organisms and a source of vitam in A in human and animal nutrition. The enzyme also converts neurospor ene to the monocyclic beta-zeacarotene but does not cyclize zeta-carot ene, indicating that desaturation of the 7-8 or 7'-8' carbon-carbon bo nd is required for cyclization. The bleaching herbicide 2-(4-methylphe noxy)triethylamine hydrochloride (MPTA) effectively inhibits both cycl ization reactions. A mutation that confers resistance to MPTA in Synec hococcus sp PCC7942 was identified as a point mutation in the promoter region of crtL. The deduced amino acid sequence of lycopene cyclase s pecifies a polypeptide of 411 amino acids with a molecular weight of 4 6,125 and a pl of 6.0. An amino acid sequence motif indicative of FAD utilization is located at the N terminus of the polypeptide. DNA gel b lot hybridization analysis indicated a single copy of crtL in Synechoc occus sp PCC942. Other than the FAD binding motif, the predicted amino acid sequence of the cyanobacterial lycopene cyclase bears little res emblance to the two known lycopene cyclase enzymes from nonphotosynthe tic bacteria. Preliminary results from DNA gel blot hybridization expe riments suggest that, like two earlier genes in the pathway, the Synec hococcus gene encoding lycopene cyclase is homologous to plant and alg al genes encoding this enzyme.